1. Field of the Invention
This invention relates to an electromechanical transducer type relay which opens and closes contacts using electromechanical transducer elements, such as electrostrictive elements or piezoelectric elements, which produce strains when subjected to an electric field.
2. Description of the Relevant Art
FIGS. 1(a) and 1(b) are views showing an example of a prior art electromechanical transducer type relay. In these figures reference numeral 3 denotes a bimorph cell which comprises a pair of joined dielectric electromechanical transducer elements, for example, of electrostrictive elements, which generates strain when subjected to an electric field. It is cantilevered at its base portion 3b by a fixed member 34 fixed to a base 5 and is electrically connected via lead members 35, 36 to a printed board (not shown) in which an electronic circuit shown in FIG. 2 is built. When a voltage is applied to bimorph cell 3, the movable end 3a of bimorph cell 3 is bent with a fixed member 34 as a supporting point in the direction of the arrow a or b.
Reference numeral 37 denotes a movable contact member which has at its end 37a a movable contact member 40 which can move to and away from fixed contacts 38, 39 which are provided to fixed terminals 41, 42, respectively. Reference numeral 43 denotes a common fixed terminal which has one end 43a to which an end 44a to a tongue 44 formed integrally with movable contact member 37 by cutting same is fixed, and has the other end 43b by which a compression spring 45 constituting a reversing spring is supported at its base 45b in a hinged manner. Compression spring 45 is engaged at its end 45a with an end 37a of movable contact member 37, so that movable contact member 37 is held in a tense state.
The base 37b of movable contact member 37 and an end 3a of bimorph cell are connected via a connection member 15 to each other. Reference numerals 46, 47 denote stops which are positioned on the return and operate sides of movable contact member 37 and abut against movable end 3a of bimorph cell 3 which is supported at one end 3b so as to restrict deformation of bimorph cell 3.
FIG. 2 shows an example of prior art drive devices of this type. In FIG. 2 bimorph cells 3 are connected in parallel via a current limiting resistor 2 between a pair of input terminals 1a and 1b. Reference numeral 4 denotes a discharge resistor connected in parallel between input terminals 1a and 1b.
In the above structure, if no voltage is applied to bimorph cells 3, bimorph cells 3 extend horizontally, as shown in FIG. 1(a). Thus end portion 37a of movable contact member 37 is biased by a compression spring 45 toward fixed contact 39, so that movable contact member 40 is moved away from fixed contact 38 and closed against fixed contact 39.
Now, when an input voltage is applied across the pair of input terminals 1a, 1b in FIG. 2, the input voltage is applied via current limiting resistor 2 to electrostrictive element 3. At this time, the input current is limited by resistor 2. Electrostrictive element 3 produces a strain in accordance with the magnitude of the input voltage and bimorph cell 3 is bent in the direction of the arrow a with a fixed member 34 as the supporting point, as shown in FIG. 1(b). When base 37b of movable contact member 37 moves beyond a change point c by bend of bimorph cell 3, the spring force of compression spring 45 acts in the reverse direction to move movable contact member 40 away from fixed contact 39 and to the fixed contact 38.
When the input voltage is shut off, the electric charges stored at electrostrictive element 3 discharge via discharge resistor 4, so that electrostrictive element 3 returns to its initial state shown in FIG. 1(a). By this return, the base 37b of movable contact member 37 moves beyond change point c, the spring force of compression spring 45 acts to move movable contact member 40 away from fixed contact 38 and to fixed contact 39.
In order to maintain the deviation of electrostrictive element 3, a voltage must continuously be applied to electrostrictive element 3. If the deviation of electrostrictive element 3 is maintained like this, electrostrictive element 3 will be disadvantageously deformed plastically. The load characteristic x of the snap action mechanism has a linearity with stroke s shown in FIG. 10, the holding forces a1, b1 of the contacts become relatively small, and large contact pressure is not obtained between movable and fixed contacts 40 and 38, and between movable and fixed contacts 40 and 39. Thus vibrations and/or shocks may cause the closed contacts to easily open to thereby result in an erroneous operation of the device. Especially when the contacts 38, 39 and 40 are worn, their pressures are further decreased, the contacts are significantly worn when large currents flow through the contacts, thereby shortening the life of the contacts.
The above is a common defect occurring when elements such as piezoelectric elements, not electrostrictive elements, which produce a strain due to application of an electric field are used as electromechanical transducer elements 3.